refactor(forms): Split up model.ts. (#45217)

model.ts is currently extremely large. This is the first step in an attempt to refactor it to be more easily navigable and reviewable. This commit breaks up `model.ts` into the following new files:

* `model/abstract_model.ts`: The remainder of the model, including the `AbstractControl` base class and helper functions which are used throughout.
* `model/form_control.ts`: `FormControl`, `FormControlOptions`, and helpers, plus the constructor and untyped friends.
* `model/form_array.ts`: `FormArray` and untyped friends.
* `model/form_group.ts`: `FormGroup` and untyped friends.

This first phase is a purely mechanical code move. There is no new code at all, and no interfaces have been separated.

PR Close #45217

packages/core/test/bundling/forms_reactive/bundle.golden_symbols.json

@@ -1115,9 +1115,6 @@
   {
     "name": "isFormControl"
   },
-  {
-    "name": "isFormGroup"
-  },
   {
     "name": "isForwardRef"
   },

packages/core/test/bundling/forms_template_driven/bundle.golden_symbols.json

@@ -1076,9 +1076,6 @@
   {
     "name": "isDirectiveHost"
   },
-  {
-    "name": "isFormGroup"
-  },
   {
     "name": "isForwardRef"
   },